Good questions
Glad you are deriving benefits.
While "condensing the shen at the upper dt" is relatively accurate verbiage, it didnt really do it for me either. Although in retrospect, yeah, having learned it now it makes sense. Here's some other analogies: from taoist yoga, "fixing the spirit at the seat of awareness," or another, "finding the source of the I-thought." So basically, bring the awareness in and keep it at the upper dt, niwan, pineal gland...the "real" "location" is for you to find, it being "neither inside, nor outside of yourself." I also came up with the correlation of setting a lamp in the middle of the room and letting it shine. This should not take any force or tension whatsoever to do, it'll give you a headache if you do it forcefully. Basically what this does is arrest the energy potential going to your cranial nerves and extending out to the senses. Usage of them requires a lot of energy, so when you withdraw the senses, it has the natural byproduct of calming those nerves and all of the resultant propagations of neural firings.
It is the combination of that + what's going on at the lower dt that leads to the manifestation of jhanas and meditative states.
For the lower dt...you will have some work to do first in harmonizing all of the structures that make you breathe. Having done so, you will also quiet the mind, because rough uneven breath stimulates the vagus (sufficient feedback will increase heartrate, breath rate) and olfactory (it is like the air meter in a car, faster flow will trigger feedback for more heartrate & breathing.) So basically your goal with breathwork is to drop the flow of air beneath the threshold of turbulence for your air passageways - so dont use your sinuses or anything above the diaphragm to breathe (may take a little while to get there, once I had that idea it basically took me a further 3 months of diligent nightly practice.)
Once the breathwork is smoothed out, once again you've attenuated signals, reduced noise in the system, and you can therefore have more clarity in picking out other signals. The lower dt is 2 inches below the navel and front:back 3:7 due to the spine. When ready you can use a slight visualization to help in perturbing the energy dynamic there - imagine a ball of light, grow and shrink with the breath from the size of a quarter to a golf ball and back. Eventually with enough other signal-noise attenuated you can get a better feel for this and you can both fix the spirit at the seat of awareness and grow/shrink the energy-ball-dynamic at the lower dt. This is a very special combination indeed!
Normal ab breath is easier to do more coherently.
Dont squeeze all the air out, it takes too much energy. Work with the middle 70% of the lung's range, because at the extreme ends it consumes energy and will make the practice less efficient. It is ok to use the front of the abdomen to assist, but as you get deeper in practice it will be less so - if you recall dr yang mentioning "the breath externally disappearing" then this is what that is referring to - troubleshooting all of this for myself led me to conclude that the fundamental 3 breath mechanisms are the diaphragm, psoas, and perineum - the front of the abdomen is really a sort of "martial modifier" that adds power when necessary. Which may sound odd at first, but to get my breaths over a minute I had to use only those 3 mechanisms and basically shut everything else off. Imagine lying down, your spine is a train track - drive the train down to the bottom of the spine (~dantien height, breath should not go as low as the genitals or as high as heart) and then back it up on exhale. So when beginning an inhale, you pull down at the lower rear attachment of the diaphragm and continue that for the duration of the inhale. This will help you rely on the diaphragm and then you wont need to buffer the incoming air with your sinuses to help balance out the pressure from uneven motion. Start relying only on those bottom 3 structures
Time them all so that they reach their energetic culmination concurrently - and time that energy potential at the lower dt. That becomes the basis for perturbing the energy dynamic there!
(This also serves as a basis for micro and macrocosmic circulations...)
Even in mildly complex systems, any outcome is the wrong thing to target, with the process being where the focus should be.